I have a question about rims based on information in another question. The person asking states that he doesn't want his rims wearing down. Is this really a problem? Assuming you don't wear the brake pads down to the metal, can the rubber pads actually wear down rims? Does it make a difference if you use aluminium, steel, or carbon rims? My rims are aluminium with tiny grooves on them. Is there anything I should look for to ensure that they aren't worn down? I have over 10,000 KM of mostly city riding on my bike.
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I have worn through quite a few rims, mostly because I live in a hilly area and I ride my bikes more than I clean them. That said, I do think that if you are to clean anything on a bike then the rims are probably what you want to be cleaning most importantly.
A couple of years ago I inflated the tyres to the value on the sidewall, walked away from the garage and heard a very loud bang. The rim had split in two on one side and I was very glad I was not in the shed at that time.
A year before that - on a small wheeled folding bike - I had a similar problem. Although I had been running some experimental brake blocks that were supposed to be 'extra good' the problem was the wheel size - smaller wheels wear out quicker because there is a smaller braking surface. A 5cm long section appeared buckled, as if I had hit a kerb, which I never do on a folding bike for obvious reasons.
I have also had some high-end Mavic rims wear through. Those had holes in the outer sidewall but they did not fail in a catastrophic way. However, they did feel 'spongy' and that was not just spoke tension.
Obviously the brakes on my 3-speed will never wear through its 50's vintage genuine-stainless-steel rims, but stopping distances are comparable to what you get with an aircraft carrier. Steel is such a no-no for rims - heavy and useless under braking.
As for how to mitigate against catastrophic rim failure, you may want to consider getting wheels with 'wear indicators' in them. A lot of the better MTBs and Road Bikes have these. Alex Rims - as found on Giant MTB's have the 'wear indicator' - this appears as a black line in the side of the rim. This should stand out against the un-anodized sidewall and when it does not you could potentially have problemos. I have not tried them myself, personally I think that it is better to have a feel for one's bike - if the wheel is too springy you know that it is on its way. The problem is rarely with the front rim (unless you do all your braking on the front). During regular cleaning you can feel the concave side of the rim, and those grooves but you don't really know if they are getting a bit too paper thin.
Returning to my comparative experience of the Mavic's that survived having big holes in them and the cheap rims that failed, I would be more inclined to go for rims like the Mavic where the outer sidewall is not 100% important to the structural integrity. The 'wear line' is a good idea but not really for me.
I managed 6000 miles - 25 miles *5 * 50 on the folding bike before the rims went, this was a hard commute with a lot of braking due to how London is a 'sliding block puzzle'. 6000 miles is an accurate guess on how long the cheapo rims on that bike lasted, that is around the 10K mark in 'clicks'. I did trial quite a few products on that bike including some 'extra good' brake blocks. The circumference braking area was probably half that of your bike, so, by rule of thumb I reckon you have got 1/3 to 1/2 way through your rims.
Nowadays I do not reach for the back brake so readily - certainly not automatically when the front one will do. For me rim wear is a real consideration.
Short Answer: Yes
Longer Answer: Yes, but maybe not for the reasons you are thinking of. Rim wear is mainly caused by dirt and grime that gets caught between the brakes and the rim, rubbing it like sand paper. The most important thing to do is clean your rims and pads as often as possible. For that matter clean your drive train as well.
As far as different materials go, it is a lot easier to bend back steel than aluminium or carbon. So if you do have a steel rim you are better off if something goes wrong. There is a tool called a dental caliper (pictured below) that can help you find out if your rim is worn down.
Here is an interesting story and some good info on bent rims.
Mountain bikes are much more prone to this, as they are commonly ridden in dirt, sand, mud, and other crud that acts like a grinding compound on the rims. Bikes ridden in more forgiving environments will likely not exhibit any appreciable wear for a very long time. My particular patrol bike is about 15 years old, with the original rims....
I can offer an anecdote that suggests that proper maintenance is a good thing!
My winter bike, which I just barely maintained, as I wore them down into the ground and then threw them out, riding all winter long in Toronto. I also only bought cheap junk bikes to do this with. Better bikes for nicer weather.
As others have noted, letting the pads wear down, or dirt between the pads and the rim can easily destroy a rim.
Twice (I should have learned) I had the brakes pierce the rim, get buried, and I flew over the handlebars. Once onto the sidewalk, once onto the road.
It was dumb, and painful. Don't do it.
I had a Mavic 222 rim fail earlier this year.
The rim was around ten years old but had spent around 6 of these unused in a garage. The rim wall split, and the inner tub popped out locking the back wheel against the frame, I just managed to keep control of the bike, as I was going quiet fast at the time.
Thank goodness it was the rear wheel.
I replaced both rims, and now only use the back brake 90% of the time. I would rather wear through the rear rim, than the front for sure.
Yes, they do, and that's what the groove on your rim is for. If your brake pads are properly adjusted, they wear a rut in the rim, giving it a concave shape. This process is imaginatively described as "concaving". As always, the late Sheldon Brown has written on the subject: look for the heading Rim Issues.
When you can't see the groove in the rim any more, it's due for replacement. If the rim is full of ridges before then, it can be smoothed by a specialist using a special kind of file, but I don't imagine this is really worth it any more.
As others have said, the brake pads themselves aren't responsible for the wear: they're pretty soft. But the penalty for this softness is that they're not so good at shifting any grit that gets between the pad and the rim. On a car's disc brakes, most grit gets removed quickly owing to the hardness of the brakes, the faster spinning, and the immense pressure, but on bike discs and especially on rims, any grit will stick around for a while, wearing gouges out of your rim. Fine particles can embed themselves in the pads to continue to wear the rim down.
This also means that the rate of wear depends heavily on the conditions you habitually ride in. If your bike is an all-year commuter bike it'll see more rim wear than a fair-weather bike. That muddy, off-road shortcut you take will be taking its toll as well. Worse yet is a proper mountain bike that's always ridden off-road on dusty and/or muddy trails.
A friend's back rim failed recently while she was riding. We're not really sure what happened, but suspect that the rim wall was thin and worn. Compare intact left side to buckled right side:
She was riding when it failed -- her tire blew off one side of her rim and her tube exploded -- scary but she wasn't hurt.
In response to comments below:
Yes like other parts they wear down. The groove on the side of your rim is likely a wear indicator. Many companies such as Alex, Weinmann and others utilize this type groove although most consumers don't realize that is it's purpose.
From Alex Wheels: "Our 0.3mm deep circumference line appears on both sidewall surfaces. When the sidewall becomes thinner the color of indicator become lighter and hard to tell the difference between the CNC machining and the indicator , it’s recommended to replace the rim."
Others do not change color, they just simply aren't a groove anymore!